1. Field of the Invention
This invention relates to the field of video insertion loss equalization. More specifically the invention relates to compensating for losses in analog video quality due to transmission over twisted pair cable.
2. Background Art
Video cables are used to convey electronic video signals from a source device such as a receiver to a destination, typically a display device. A cable is supposed to accurately convey the signal, however, losses accumulate along the cable path because of imperfections in a transmission cable. These imperfections are not necessarily due to manufacturing but due to the fact that a cable is a physical device and most physical devices exhibit some losses when a signal is conveyed through them. Thus, the longer the cable length, the more losses accumulate. The accumulated transmission loss is known by those of skill in the art as cable insertion loss. Of course, other devices such as switches and splitters in a video transmission path contribute to the total video loss however only transmission cable insertion losses are considered in this specification.
Video may be transmitted either in digital or analog formats. For digital video transmission such as computer video, cable insertion loss is generally not an issue because the digital signal can be recovered so long as discernable digital pulses are received at the receiving station. However, for analog signals such as NTSC (National Television Standards Committee) video, the signal is just voltages, and voltages are affected by wire length, connectors, heat, cold, and other conditions. This degrading effect on the video signal caused by the transmission cable length is known as cable insertion loss.
Analog video such as the C-Video, S-Video, or YUV (or YIQ) specifications may be available in any of the different color models. A color model (also color space) facilitates the specification of colors in some standard, generally accepted way, for example, the RGB color model where R is for the red component, G is for the green component, and B is for the blue component. For high-resolution analog video, each color component is usually transmitted separately from the receiver to the display device. Thus, each color component must be examined for cable insertion losses.
A coaxial cable is typically used for transmission of high resolution (i.e., broadband) video because of its superior performance over twisted pair cable. However, coaxial cables are more expensive and difficult to install compared with twisted pair cables. Historically, the significant differences between coaxial cable and twisted pair cables limited twisted pair transmission to low-resolution video (i.e., less than 10 MHz). However, twisted pair has one distinct advantage over coaxial, i.e., cost/performance ratio. Dollar-for-dollar, twisted pairs are significantly cheaper than coax or fiber (i.e., fiber optic cable) to buy and install. And, while the coax and fiber offer some advantage in bandwidth, this advantage may be offset with proper compensation. Also, standard twisted pair, i.e., UTP/STP (Unshielded/Shielded Twisted Pair) cable contains four pairs in a single cable so that the actual cost per pair is one-quarter of the per-foot price. Therefore, transmission of broadband analog video over twisted pair cable is highly desirable.
The idea of high-data rate twisted pairs started with an attempt to organize cables into levels. In the process, it was recognized that twisted pairs could be made into higher grade data versions. These data-grade twisted pairs come in two flavors: twisted pairs commonly called UTP (unshielded twisted pairs) and STP (shielded twisted pairs). By far, most of the domestic data installations today employ UTP.
In the mid 1980's, twisted pair technology began to emerge which could transmit 2 Mbps, then 4 Mbps (the original IBM data rate), and then 10 Mbps. As these data rates increased, it became apparent that some way of indicating the performance of the cable was needed. It was then that the system of “Levels” was suggested. The TIA/EIA, two groups that set standards for the data industry, adopted the plan and separated the data rates and other parameters into “Categories”, such as Category 3, 4, and 5. Category 4 cables are becoming extinct. Each higher numbered category has more stringent requirements with higher data rates and higher performance than the previous category. The specifications for Categories are given in TIA/EIA 568A.
Current twisted pair cables look identical to the plain old telephone service cable. They use the same color code and come in many of the same pair counts and use the same gage conductors. However, the specifications they are made to, the materials used to make them, and the requirements to connect them, become more and more critical as the data rate increases.
Twisted pair cable has many current common uses, such as in low-resolution video display systems and in business and corporation local area networks. With respect to low frequency performance of video displays, twisted pair cable carries horizontal line time, at frequencies that range from 15 to 50 KHz, for example. In order to display a complete video picture, each horizontal line sweep of the electron beam needs to be coordinated with color information and vertical line information. In computer monitors and displays, the picture information is displayed in “progressive” scan, i.e., one horizontal line right after another. This gives these displays excellent detail (which is required in the computer field) but requires double the amount of information as an interlaced analog broadcast receiver to display a picture at the same resolution.
Timing pulses can be used to ensure that each step in the process of transmitting video information occurs at the right time. However, although the video signals may be properly timed, all transmission lines have a characteristic signal loss in any system for which they are utilized. This characteristic signal loss in response to an applied electromagnetic wave along a cable is the insertion loss previously discussed. The efficiency of a transmission line carrying such a signal affects graphics image response and the degree of streaking across a picture screen. Low frequency streaking (or smear) across a video display screen, also known as picture effect, creates an overall appearance of detail loss and discontinuity critical to an observer's eye.
There are three key problems using twisted pair (e.g., UTP) for analog video. First, the majority of video equipment uses coaxial connectors, usually BNCs. The second is the output impedance of the coaxial systems is 75 Ω (ohms) while UTP has an impedance of 100Ω. The third difference is that UTP being twisted pairs is a “balanced line” system while coax, which has only one shielded conductor, is “unbalanced”. Prior art devices attempt to solve these problems with a device called a “balun”. Balun, in fact, means BALanced to Unbalanced. The balun is a small box which contains a transformer and other matching components allowing the signal to be converted from 75Ω impedance to 100Ω impedance. It can also change the signal from unbalanced to balanced; using a BNC connector (commonly used for coax) on one side of the box (i.e., input side) and an RJ-45, the most common UTP connector, on the other side (i.e., output side). However, baluns can only handle very narrow bandwidth and no DC component (basically a bandpass filter).
Balance is also a critical parameter. The nature of a balanced line means that the two conductors in the twisted pair are identical in length. The more identical they are, and the closer they are together, the easier it is for the balun to reject noise and interference generated outside the pair. When noise hits both conductors, and a resultant noise “pulse” is generated in both wires, the more identical the noise on each wire at the end of the cable, the greater the noise rejection can be (‘common mode rejection’) in a balanced line. The less identical, for example, standard telephone service lines are often very unequal, the more noise will get through.
Signal reconstruction at the far end depends on balance. Twisted pair is a difference technology. The effect in the receiver is not dependent on the voltages on the wires but the difference in the voltages on the wires. Currents in the wires that are identical will not contribute to the received signal. Practical circuits have limits in voltage range and current delivery to compensate for common mode voltages and currents. The difference in chassis ground voltages can be on the order of 5 volts at 500 feet in most industrial locations. Digital circuitry has large currents (20 to 50 amps in their ground planes causing 200 mV noise). Thus, to keep common mode signals from being converted into difference signals the impedances experienced by each line must be identical.
Common UTP/STP Properties
ACR is “attenuation-to-crosstalk ratio”. By subtracting the attenuation from the crosstalk, a number is generated which can indicate the overall performance of a cable. Positive ACR, especially at high frequencies, can be an indicator of cable performance. On the other hand, it is possible to dramatically improve crosstalk, and thereby improve ACR, by unusually tight twisting of the conductor pairs.
Skew or Delay Skew is timing differences on a multi-pair cable. Skew is especially interesting when using more than one twisted pair to simultaneously deliver data, which is especially important with analog video transmission. In such systems, it is essential that the signals arrive at the other end of the cable at the same time. For example, the R, G, and B components of a high-resolution video are transmitted on separate wire pairs.
Transmission Lines
The next level of approximation deals with the behavior of the twisted pair line when the length of the line exceeds the longest wavelength of interest, which is almost always the case with video transmission. Under these conditions the classical RLC telephone equations apply for frequencies of the energy propagated throughout the line.
There are three frequency regions of interest: The diffusion region in which Resistance (i.e., R) per unit length is greater than Inductance (i.e., L) per unit length, the standard transmission telephone equation region, and the skin effect region. In the diffusion region (i.e., where R dominates over L), the classical telegraph equations apply. The Skin effect region, which is above 5 MHz with CAT5 twisted pair, is characterized by a loss that increases in proportion to the square root of frequency. In the limit the telephone equations devolve into the telegraph equations. Thus, the diffusion region and the telephone region may be combined and solved as a diffusion line problem, which can be approximated by several RC networks distributed over several decades of frequencies.
A typical insertion loss response indicates that for a given twisted pair cable, as much as two thirds of the insertion loss occurs in the first few megahertz of its operating frequency range. This loss is mainly due to diffusion in the wire. The magnitudes of each of these forms of signal loss are dependent on the effective capacitance of the wire. The combination of a long cable run with higher direct current (DC) loss coupled with the changing resistivity at low frequencies due to diffusion effect causes the transmission line to appear more like an R-C time response. This time response change causes medium to large details on the image to create a trailing streak as current is changing direction in the cable. The condition is commonly referred to as short-time or line-time distortion when seen in a video processing circuit which has a long response time constant, or time necessary for the system to reach steady state. The R-C like response time constant is related to the distributed capacitance times the distributed resistance, and it is generally accepted that the current becomes constant after five times the distance integral of this time constant. It is this long time constant that degrades the image to an unacceptable point in most cases.
Impedance
Impedance indicates the ability of a data cable to attach and transfer energy from one box to another. The impedance of the cable is determined by the physical construction of the system. The TIA/EIA standard for Category 5 is 100Ω±15Ω. Some Category 5 cables meet this spec. Others require the use of a smoothing formula called “Zo-fit”. This allows manufacturers to ignore rapid changes in impedance. While many Enhanced Category 5 manufacturers say that their cable is “tested to 300 MHz” (or even up to 400 MHz), they offer no data (impedance, crosstalk, attenuation, ACR, skew etc.) at those high frequencies. Impedance variation can often be 100Ω±50Ω, rendering these cables virtually useless for uncompensated video transmission.
Bandwidth
Bandwidth is the range of frequencies available to be used for signal carrying. It is the “size of the tunnel”. Bandwidth is a measure of the signal-carrying capacity of different cables. For broadband video, uniform amplitude and delays across colors are required.
Cable insertion loss can be characterized as a function of frequency (i.e., rate of the video input). FIGS. 1A and 1B illustrate such characteristics for a 300 feet Category 5 twisted pair cable. FIG. 1A shows the square root of frequency characteristics of the skin effect loss, while FIG. 1B is a more traditional view using a log-log scale. Insertion loss is specified in decibels (dB). As shown in the illustrations of FIGS. 1A and 1B, the insertion loss is approximately 0 dB when the input frequency is near zero, i.e., the output will be approximately equal to the input if the input is constant for a long period of time. In addition, the insertion loss increases as the input frequency increases. In general, insertion loss increases as the cable length over which the analog video is transmitted increases. This is why compensation is generally not needed and usually not applied for short cable runs, e.g., six feet or less.
Several compensation techniques may be used to compensate for cable insertion losses, however, in broadband systems where signals range from the steady state to several hundred megahertz, it is nearly unfeasible in terms of cost to compensate the entire frequency spectrum (typically 0–300 MHz). It is particularly problematic because of the shape of the frequency response characteristics of the insertion loss which shows rapid drop off in the low frequency range (0–10 MHz) and then followed by a shallow drop over the remainder of the frequency range (see FIG. 1A). The insertion loss characteristics at the low frequency region are due to diffusion in the cable. Cables of different type or length have different diffusion rates. Current industry methods effectively compensate for the high frequency end of the spectrum but not the low frequency end because of the complex frequency response characteristics associated with insertion loss of twisted pair cables. The problem with compensating for high frequency losses without compensating for the low frequency losses in video transmitted over long twisted pair cable is that people are more tolerant to seeing less sharp pictures (high frequency effects) so long as they can see the information. However, people are least tolerant of low frequency anomalies which are characterized by distortion or smearing type phenomenon across the video screen.
FIG. 2 is an illustration of a setup for a video display from a source signal to a destination display device. The input video signal is VIN, and the output video signal displayed on display screen 200 is VOUT. As discussed earlier, video input signal VIN is an analog voltage signal thus video amplifier 204 is used in a video receiver to condition the voltage to the level desired by the display device. Generally, video amplifier 204 conditions the analog video input signal to compensate for transmission line loss such that the proper video signal reaches display device 200. Block 208 is the operational amplifier gain and block 206 is the current feedback gain (generally representing resistive dividers). The feedback is from the output of the operational amplifier through resistors (represented by block B 206) to the negative input terminal of the operational amplifier. Thus, in this configuration, the transfer function between the input and output voltage of the video amplifier is A/[1+AB].
Twisted pair cable line 202 represents the total cable length the video must travel from the input source to display device 200. Thus, assuming there is no change in voltage at VGA to TP Converter 204, FIG. 1 would represent the frequency responses between input signal VIN and output signal VOUT for a 300-foot twisted pair cable. FIG. 3 shows the transient response characteristics of a fixed length twisted pair cable. Using FIG. 2 as an illustration, a step input (e.g., 300), which is characterized as having frequency content from zero to approximately infinity, is introduced at VIN. The transient response output VOUT (e.g., 310) is shown in FIG. 3. Region 320 represents the low frequency diffusion effects while region 330 represents the high frequency skin effects of the transmission cable. The effect of the low frequency characteristic of the transmission cable is also evidenced by the large rise time, i.e, time it takes the output to reach 90% of steady state. The gradual rise of the response (i.e., 310) over a long period of time appears as a smear or shadow effect on the uncompensated video signal at display 200. Large rise time is synonymous with low bandwidth. Thus, the twisted pair cable introduces a low bandwidth filter effect on the input video signal passing through it. Generally, rise time increases with decreasing characteristic bandwidth and longer cable lengths tend to produce lower characteristic bandwidth.
To illustrate the filter effect of the twisted pair cable, FIG. 4 shows a low pass filter being used as an example to represent the effect of cable line 202 of FIG. 2. That is, the low-pass filter represents the transfer function between the input and output of the twisted pair cable. Using low pass filter 400 with bandwidth ω representing the characteristic bandwidth of the cable length, VOUT is given by the equation:
      V    OUT    =            A              [                  1          +          AB                ]              ⁢          1              [                                            1              ω                        ⁢            s                    +          1                ]              ⁢          V      IN      
Thus, the goal is to make bandwidth ω as large as possible so that the frequency response between the input and output videos remains as flat as possible to a frequency high enough that any distorting effects caused by the transmission line are not discernable by the human eye. One way to compensate for the effect of the low bandwidth ω of the cable characteristics is to include the twisted pair cable line loss in the feedback loop as shown in FIG. 5. Thus, instead of the current feedback originating directly from the output of video amplifier 504, cable characteristic 400 is included in the closed loop and thereby compensated for in video amplifier 504. The resulting transfer function between input VIN and output VOUT is given as follows:
      V    OUT    =            A              [                  1          +          AB                ]              ⁢          1              [                                            1                                                (                                      1                    +                    AB                                    )                                ⁢                ω                                      ⁢            s                    +          1                ]              ⁢          V      IN      
Thus, the new bandwidth between the input and output voltage is increased by the factor (1+AB) and thus directly controllable by the gains chosen for the video amplifier feedback. However, this implementation would require feedback from a long (e.g., 200 feet) cable back to the video (i.e., operational) amplifier 504 which is located at the source. Firstly, this is impractical and secondly there is a significant amount of time delay involved in feeding back from the long cable runs. Long feedback delays tend to cause loop instability and thus limits loop gains that can be used in the video amplifier. The most practical way of compensating for the cable insertion loss characteristics is to duplicate those characteristics in the feedback path as shown in FIG. 6 thus eliminating the time delay involved in feeding back from a long cable run.
FIG. 6 illustrates the most practical way of compensating for cable insertion loss. Mathematically, this provides the same results as FIG. 5 above, i.e., the loop bandwidth is increased by [1+AB]. Assuming the gain B is unity, then VOUT approaches VIN as A increases. In a perfect situation, Block 600 replicates the inverse of the cable characteristics. However, cost prohibitions may make it impractical to create analog circuitry for block 602 in video amplifier 600 that duplicates the cable insertion loss characteristics 400 in a wide frequency range as shown in FIG. 1. Thus, prior art implementations tend to only compensate for the high frequency losses by adding “peaking” compensation, i.e., pole-zero pair to boost the response at the high frequencies.